Glass fibers, by reason of their relative chemical inertness, high temperature stability, and moderate cost have found particular use as a thermal insulation for buildings, industrial equipment, and pipes. The growth of the insulation market for glass fibers has rapidly increased in recent years by reason of the demand for additional insulation in both new and existing buildings due to ever increasing fuel costs.
Glass fibers for insulation use are typically formed from a high-silica, soda-lime glass containing on the order of at least about 60 percent by weight of silica and about 12 percent or somewhat more of sodium as the oxide, together with significant amounts of lime, e.g. 5-10 percent of CaO.
A glass commonly employed in the manufacture of glass fibers for insulation use has the approximate composition:
TABLE I ______________________________________ Component Percent by Weight ______________________________________ SiO.sub.2 61.2 Al.sub.2 O.sub.3 4.0 Fe.sub.2 O.sub.3 0.2 CaO 8.0 MgO 3.3 B.sub.2 O.sub.3 7.5 Na.sub.2 O 14.25 K.sub.2 O 1.3 ______________________________________
Such glass composition has a melting point on the order of about 1004.degree. C.
As is well known in the glass-making art, control of the softening and melting temperatures to provide the glass composition with greater fluidity is important in the formation of glass filaments. In order to promote melting and dissolution of the raw materials in the charge and to reduce the melting point of the glass, fluxes such as sodium carbonate, calcium fluoride, and sodium nitrate have been added to the charge of glass-forming components.
For example, in the manufacture of high-silica glasses, examples of which are soda-lime, borosilicate, and aluminosilicate glasses, the solid constituents, in the proper proportions, are dry blended, as for example in a pan type mixer. Small amounts of water may be added for further wet blending. When the intimately mixed batch is charged into a heated furnace, a series of melting, dissolution, volatilization, and redox reactions takes place between the materials, in a particular order and at the appropriate temperatures. The purposes of the flux is to accelerate dissolution of the more refractory (higher melting) grains, such as sand. However, using a flux such as sodium nitrate, oxides of nitrogen are generated during the glass-making operation. Such oxides are highly corrosive to plant equipment and are air pollutants in the plant effluent. Therefore, expensive pollution control measures must be taken when using sodium nitrate as a flux.
The use of lithium carbonate as a flux in glass-making has been suggested. For example, U.S. Pat. No. 4,066,466 discloses a process in which lithium carbonate is added as a flux to standard compositions used in the manufacture of glass fibers, such as "621" or "E" glass compositions, in place of calcium fluoride, to reduce the liquidus and softening temperatures of the glass composition. Although the use of such compound reduces fluorine emissions in the stack gases of glass furnaces, such use presents other problems.
It has been determined that when lithium carbonate is substituted for a flux comprising a mixture of sodium nitrate and sodium sulfate (3:1 weight ratio), the fluidity of the glass batch is improved. Unfortunately, it was observed that after about three days' operation, a crust of unmelted raw materials in excess of one foot in thickness built up, and the rate of dissolution of the raw mix was greatly diminished. It is believed that such undesirable results are attributable to the high liquidus temperature of lithium carbonate, which is about 722.degree. C. By the term "liquidus temperature" as used herein is meant that temperature at which the composition begins to solidify upon cooling after being heated to a temperature at which the composition is in the liquid phase.
It is an object of the present invention to provide a novel flux composition which reduces the dissolution temperature of raw materials used in the glass-making operation.
It is another object of this invention to provide an improved process for making glass of greater fluidity particularly suitable for spinning of glass fibers for insulation or other use.
A further object of this invention is the provision of a novel process for making glass without use of a flux which decomposes to form corrosive materials or atmospheric pollutants.
Other objects and advantages of the invention will become apparent from the following description and accompanying drawing, which is a binary freezing-point diagram for mixtures of lithium carbonate and sodium sulfate.